The effects of hypoxia will be studied morphologically in the lung, with neurophysiologic techniques in the brain stem, and with behavioral and cardiopulmonary techniques in intact animals, in three units simultaneously. A) Intrathoracic petechiae and pulmonary edema will be examined in rats with intravascular casts to determine the site of leakage; the development of leaks will be related to the pathophysiology of hypoxia. B) The medullary gasping center will be searched for with neurophysiologic and histologic methods in cats and monkeys, and its function in extreme hypoxia will be related to the function of the rhythmic respiratory centers. C) The effects of maturation, sleep state, and hypoxia on the ability to arouse after primary apnea will be studied in infant monkeys. Moderate hypoxia will be studied in infant monkeys to determine if depression of ventilatory drive is produced in the immature animal. Several classes of drugs will be studied in dogs and baboons for their ability to improve arousal, stimulate ventilatory drive, or reduce the effects of hypoxia.